The present invention relates to a pneumatically operated screw driver preferably used for screwing a threaded fastening member to a woody material or the like. More particularly, the invention relates to the screw driver provided with a mechanism for attaching and detaching a drive bit.
Various pneumatically operated screw drivers have been conventionally proposed. According to a typical arrangement of the pneumatically operated screw drivers, a drive bit is rotated by an pneumatic motor to screw a threaded fastening member. For example, U.S. Pat. No. 6,026,713 discloses a screw driver including an pneumatic motor in which a rotor is rotatable in response to the pressure of pressurized air. A cylindrical rotary member is connected to the pneumatic motor for causing a rotation in synchronism with the rotation of the rotor. A rotary slider is slidable in the axial direction along the inner cylindrical wall of the rotary member. A rotational force transmitting mechanism is provided for transmitting the rotation of the rotary member to the rotary slider. A shaft has one end fixed to the rotary slider and the other end equipped with a piston and a drive bit holder. A rotational and axial motion of the rotary slider is transmitted to a drive bit held in the drive bit holder. A cylinder guides the axial slide movement of the piston responsive to the pressure of pressurized air applied on a pressure-receiving surface of the piston.
As shown in FIG. 9, a piston 313 integrally provides a drive bit holder 313A in which a hexagonal hole 313a is formed. An upper end of a drive bit 316 is formed with a hexagonal shaft 316A that is fitted in the hexagonal hole 313a. This connection links rotation of the drive bit 316 and the piston 313 so that the drive bit 316 does not rotate idly with respect to the piston 313. Further, the hexagonal shaft 316A is formed with an annular bit groove 307, and a plurality of holes are formed in the bit holder 313A for receiving therein balls 304. A biasing ring 305 is provided around the bit holder 313A for pressing the balls 304 radially inwardly toward the axial center of the hexagonal shaft 316A. Thus, each ball 304 is engaged with the annular bit groove 307. This prevents the drive bit 316 from pulling out from the bit holder 313A.
Because the pneumatically operated screw driver presses the drive bit 316 down against a screw 18 while rotating the drive bit 316, the tip end of the drive bit 316 that directly connects the screw 18 can be frequently damaged by friction. The drive bit 316 needs to be replaced each time it is damaged.
To replace the drive bit 316, the drive bit 316 is pulled out from the piston 313, and a new drive bit 316 is then inserted in its place. However, before performing this bit changing operation, the user must first undo attachment bolts (not shown) from a casing 301 to remove a nose 370. Then, the user grasps the tip of the drive bit 316 using a tool, such as a pair of pliers, and pulls the drive bit 316 off the piston 313 so as to forcibly displace the balls 304 from the annular bit groove 307. Next, the user inserts the new drive bit 316 and reattaches the nose 370 to the casing 301.
However, these operations for replacing the drive bit 316 are complicated, troublesome, and time-consuming. The disassembling operations of removing the attachment bolts and the nose 370 are not only troublesome, but can allow dirt and dust to enter into the area around a cylinder 315 while the nose 370 is removed. This dirt and dust can become the cause of break downs.
Sometimes during use of the screw driver, the tip of the drive bit 316 can slightly fuse to the cruciform hole in the head of the screw 18. As a result, the drive bit 316 tends to cling to the screw 18. Since the drive bit 316 is supported on the piston 313 merely by urging force of the biasing ring 305 against the balls 304. Therefore, if the drive bit 316 clings to the screw 18, then the drive bit 316 can pull off the piston 313 when the piston 313 returns to its initial position. To avoid this problem, the biasing force of the biasing ring 305 can be increased so that the drive bit 316 does not pull off so easily. However, this is not a desirable solution because the drive bit 316 would be harder to pull off when the drive bit 316 needs to be replaced.